Page 354 - Civil Engineering Formulas

HIGHWAY AND ROAD FORMULAS 285

DESIGNING HIGHWAY CULVERTS

A highway culvert is a pipelike drainage facility that allows water to flow
under the road without impeding traffic. Corrugated and spiral steel pipe are
popular for culverts because they can be installed quickly, have long life, are
low in cost, and require little maintenance. With corrugated steel pipe, the seam
strength must be adequate to withstand the ring-compression thrust from the
total load supported by the pipe. This thrust C, lb/ft (N/m), of structure is

S
C (LL DL) (11.25)
2
2
2
where LL live-load pressure, lb/ft (N/m )
2
2
DL dead-load pressure, lb/ft (N/m )
S span (or diameter), ft (m)
Handling and installation strength must be adequate to withstand shipping
and placing of the pipe in the desired position at the highway job site. The han-
dling strength is measured by a flexibility factor determined from
D 2
FF (11.26)
EI
where D pipe diameter or maximum span, in (mm)
2
E modulus of elasticity of the pipe material, lb/in (MPa)
I moment of inertia per unit length of cross section of the pipe wall,
4
4
in /in (mm /mm)
The ring-compression stress at which buckling becomes critical in the inter-
action zone for diameters less then 126.5r/K is
2
KD
f c 45,000 1.406 (11.27)
r
For diameters greater than 126.5r/K,
12E
f c (11.28)
(KD/r) 2
2
where f buckling stress, lb/in (MPa)
c
K soil stiffness factor
D pipe diameter or span, in (mm)
4
4
r radius of gyration of pipe wall, in /in (mm /mm)
2
E modulus of elasticity of pipe material, lb/in (MPa)
Note: For excellent sidefill, compacted 90 to 95 percent of standard den-
sity, K 0.22; for good sidefill, compacted to 85 percent of standard density,
K 0.44.